A technique of encrypting a video signal for transmission and decrypting the encrypted signal at a receiver end for display is practiced in a viewpoint of preventing copying of the video signal to protect copyright, or keeping fairness in charged broadcasting.
However, if the receiver is modified and the decrypted video signal is taken out to make a copy thereof, the above-mentioned copyright protection and fairness cannot be ensured.
FIG. 10 is a block diagram illustrating two casings containing an encrypted-video description adapter for decrypting an encrypted video signal, and a video display unit, and the internal structures thereof, which is disclosed in Japanese Published Patent Application No. 2003-152701 (Patent Document 1). In FIG. 10, reference numeral 320 denotes an encrypted video decryption adapter, numeral 350 denotes a video display unit, numeral 324 denotes a signal processing circuit, numeral 340 denotes an AC/DC conversion circuit, numeral 337 denotes a wattmeter or an ammeter, numeral 338 denotes a reference value, numeral 339 denotes a comparator, numeral 341 denotes a SW, numeral 328 denotes a decrypted analog video signal, and numeral 355 denotes a power code for supplying a voltage to the video display unit 350. Further, numeral 335 denotes an AC connected included in the encrypted video decryption adapter 320, which is connected to the wattmeter or ammeter 337.
In FIG. 10, the encrypted video decryption adapter 320 and the video display unit 350 are contained in separated casings, and a power to the video display unit 350 is supplied from the AC connector 335 in the encrypted video decryption adapter 320 through the power code 355.
If the decrypted analog video signal 328 is not inputted to the video display unit 350 but is connected to another video recording device, a voltage or a current measured by the wattmeter or ammeter 337 shows a result different from a voltage or a current to be consumed by the video display unit 350.
As the result, an output of the comparator 339 turns off the SW 341, and a DC voltage produced by the AC/DC conversion circuit 340 is not transmitted to the signal processing circuit 324, whereby the decryption operation is halted.
As described above, when the decrypted analog video signal 328 is not inputted to the video display unit 350 but is connected to another device that is not shown in FIG. 10, the decryption is halted, whereby copyright protection can be achieved.
Further, in a digital television receiver, generally, enhancement in performance of a digital decoder unit for decoding digital data is faster relative to a display panel unit for displaying video, resulting in a problem that the digital decoder unit becomes obsolete. Therefore, it is expected that a structure which enables version upgrade of hardware by exchanging only the digital decoder unit is demanded. When the user is allowed to perform version upgrade of hardware, the user opens the cover of the electronic equipment to exchange the electronic device in the casing.
The conventional opening/closing detection of the cover of the electronic equipment is aimed at only preventing falsification of the equipment. For example, as disclosed in Japanese Published Patent Application No. 2002-7215 (Patent Document 2), falsification of the equipment is judged by one-time detection to make the equipment unusable.